1. Field of the Invention
The present invention relates to a vehicle motion control apparatus, particularly relates to an apparatus for performing a vehicle stability control in response to state of a tire on a road.
2. Description of the Related Arts
In general, when a vehicle is moving with a motion in a lateral direction or yawing motion, a rotating plane of a wheel is not directed to a moving direction of a vehicle on a road, so that a so-called lateral slip is caused. As a parameter or factor indicative of that state of the wheel, a slip angle, or wheel slip angle is employed. According to a book entitled “Vehicle Dynamics and Control” written by Mr. Masato Abe in Japanese, and published by Sankaido Co., Ltd., on May 31, 1994, it is defined that an angle between a moving direction of a wheel and a rotating plane of the wheel (a direction to which the wheel is oriented) is named as “slip angle”. In that book, it is described that in the case where the wheel slip is being caused, “lateral force” is produced in a direction perpendicular to the rotating plane of the wheel, in addition to driving force, braking force or the like, and it is further described that a component of the lateral force in a direction perpendicular to the moving direction of the wheel is named as “cornering force”.
With respect to the lateral force as described above, a parameter or factor called as a lateral force utilization ratio, or lateral G utilization ratio has been used in a variable gear ratio steering apparatus for a vehicle, which is disclosed in Japanese Patent Laid-open Publication No. 11-99956 (corresponding to U.S. Pat. No. 6,155,377), and which was aimed to prevent a steered wheel from being over-steered. According to the apparatus as disclosed in that Japanese Patent Laid-open Publication, a road coefficient of friction μ is estimated at the outset, to obtain the lateral force utilization ratio. The lower the road coefficient of friction μ is, the more a cornering power Cp (value of the lateral force per the slip angle of one degree) of a tire is decreased, whereby a reaction force of a rack shaft applied from a road with a certain steered angle is decreased according to the road coefficient of friction μ. Therefore, it is described that the road coefficient of friction μ can be estimated by measuring a steered angle of a front wheel and an actual reaction force of the rack shaft, and comparing the actual reaction force of the rack shaft for the steered angle with a reference reaction force of the rack shaft provided in advance as an inside model. Furthermore, a maximal lateral force is obtained by providing an equivalent friction circle on the basis of the road coefficient of friction μ, and subtracting a part of the frictional force caused by a longitudinal force, and its ratio to the present lateral force is identified as the lateral force utilization ratio (“side force utilization ratio” in the corresponding U.S. Patent). It is also described that, with a lateral G sensor installed, the lateral G utilization ratio can be provided on the basis of the lateral G detected by the lateral G sensor.
In contrast, a parameter reflecting a property of a wheel as a pneumatic tire is explained in prior papers entitled “Estimation of Grip State Based on Self Aligning Torque and Its Application to Enhance Vehicle Stability”, written in Japanese by Mr. Yuji Muragishi et al. including five inventors of the present invention, and distributed at Spring seminar held by Society of Automotive Engineers of Japan, Inc., on May 22, 2003. That is, such a parameter that indicates a margin for the tire with its cornering force being reached to its limit, in other words, the parameter that indicates the margin up to the limit how much of maximal force can be produced by the tire, has been defined as a lateral grip margin. And, it is explained that the lateral grip margin can be estimated on the basis of a self aligning torque and a reference self aligning torque, calculation of which are explained in detail in the prior papers, and therefore omitted herein. Furthermore, in the prior papers, a steering and braking control using the estimated parameter has been proposed, and a result evaluated the improvement of performance in the vehicle stability control has been disclosed. It is stated that the vehicle stability control can be initiated from such a state that the tire is getting close to its limit zone with a margin being still remained. As an example applied to a steering system, there is disclosed an example with the estimated result of the lateral grip margin applied to a variable control of overall steering gear ratio. And, as an example applied to a braking system, there is disclosed an example with the estimated result of the lateral grip margin applied to a decelerating control. And, it is described that a future issue to be considered is a total control for combining the steering system and the braking system.
With respect to the property of wheel as the pneumatic tire discussed in the aforementioned prior papers, it is also described in the above-described book such that among various wheels such as a wheel with a pneumatic rubber tire, wheel with a rigid rubber tire and an iron wheel, the wheel with the pneumatic rubber tire is capable of producing the maximum force, with reference to the relationship between the slip angle and the cornering force for each wheel. Then, the wheel with the pneumatic rubber tire has been simply referred to as “tire”, and explanations have been made about the force applied to the tire with the lateral slip being caused, and the property of the force, and further about the self aligning torque as described above.
The above-described lateral grip margin is clearly distinguished from the lateral force utilization ratio, or lateral G utilization ratio as described in the Japanese Patent Laid-open Publication No. 11-99956. According to the apparatus as disclosed in that Publication, a road coefficient of friction μ is estimated, to provide the maximum lateral force that could be produced on the road. This road coefficient of friction μ is estimated on the basis of a reliability of the cornering power Cp (value of the lateral force per the slip angle of one degree) on the road coefficient of friction μ. However, the cornering power Cp relies not only on the road coefficient of friction μ, but also a configuration of the area of the road contacting the tire (its contacting length and width to the road), and elasticity of the tread rubber. For example, in the case where water exists on the tread surface, or the case where the elasticity of the tread rubber has been changed due to wear of the tire or its temperature change, the cornering power Cp will vary, even if the road coefficient of friction μ is constant. In the Japanese Patent Laid-open Publication No. 11-99956, however, nothing has been considered about the characteristic of the tire which constitutes the wheel. Therefore, the lateral force utilization ratio or lateral G utilization ratio as described in the Japanese Patent Laid-open Publication is essentially different from the parameter or factor indicative of lateral grip margin (hereinafter, referred to as grip factor) as described in the aforementioned prior papers, but can be included in the parameter indicative of lateral margin for the tire which can be provided for use in the vehicle motion control apparatus according to the present invention.
With respect to parameters for performing the steering control for preventing the vehicle behavior from being changed, in addition to the lateral force utilization ratio as described above, the parameters such as the one provided on the basis of the slip angle of the wheel can be used. According to the present application, therefore, those may be served as the parameter indicative of lateral margin for the tire, as well as the grip factor as explained above, provided that those parameters can be provided for the decelerating control, while there may be differences in effects among those parameters. The decelerating control is such a control that decreases vehicle speed irrespective of operation of a vehicle driver, and may be achieved by a braking pressure control device, a throttle control device or so-called fuel injection control device for use in an engine, a shift control device for controlling a shift gear ratio or the like, thereby to decrease the vehicle speed. In the case where the steering control is performed to control the tire angle (steered wheel angle) thereby to achieve the vehicle stability control, on the basis of the above-described parameter indicative of lateral margin for the tire, it will not deteriorate a feeling to a vehicle driver so much, because it will not cause a large change in vehicle behavior, but its effect will be rather small. On the contrary, in the case where the decelerating control such as the braking control is performed to decrease the vehicle speed on the basis of the above-described parameter indicative of lateral margin for the tire, thereby to achieve the vehicle stability control, its effect for the vehicle stability control will be large, but it is likely that a different feeling due to the change in vehicle behavior will be given to the vehicle driver. Therefore, it is important to have the steering control and the decelerating control be combined appropriately in view of their advantages and disadvantages, thereby to achieve the vehicle stability control effectively, with the different feeling being caused as small as possible.